Most likely the little check valve in the top of the loop. With low pressure in the main exhaust hose, the little valve would inhale every time the muffler burps water.. you could try removing the valve and cleaning it carefully. The valve usually unscrews easily.. another way might be to eliminate the little valve . You have to have a high place to vent to , higher than the vented loop by a foot or more.. The little valve would be replaced with a hose barb fitting that would screw on in place of the valve. from that hose barb, a piece of small diameter tubing (maybe 1/4") would be led to a high point where it would vent water when the engine is running under load..
You could try a different brand vented loop .. though that is an expensive trial-and-error game to play.
Some folks even use an electric solenoid valve to close the loop vent when the engine is running..